Ion exchange is an important separation technique that is widely used for water and waste treatment, biochemistry, medicine and environmental protection. The efficiency of separation depends on many factors, including the selectivity of the adsorbent in use. Inorganic ion exchangers and adsorbents, due to properties such as chemical and thermal stability, resistance to oxidation and unique selectivity to certain ions, have definite advantages in comparison with traditionally used organic resins. Inorganic ion exchangers are able to operate in extreme conditions (high temperature or strong radiation fields, in the presence of organic solvents and/or oxidants and in a great excess of competitive ions), where organic resins fail to work efficiently. Among the known inorganic adsorbents, hydrous zirconium oxide (HZO) attracts special attention. HZO shows amphoteric properties and can behave as a cation exchanger in alkaline media or an anion exchanger in acidic media. However, the practical use of HZO is relatively limited because of several factors, among them being poor reproducibility of ion exchange properties and a strong dependency of adsorption performance on moisture content.
There are several methods for making amorphous hydrous zirconium oxides (A. Ruvarac, Group IV Hydrous Oxides—Synthetic Ion Exchangers, in the book, Inorganic Ion Exchange Materials, Ed. A. Clearfield, CRC Press, 1982, pp.141-160). Typically, in the first stage hydrolysis of a zirconium salt takes place, which is followed by condensation of primary particles, formed with hydrated polynuclear species. Hydrolysis of zirconium salts can be achieved by boiling an aqueous zirconium salt for a long period of time or by neutralization with alkali (e.g., LiOH, NaOH, KOH and NH4OH). Freshly prepared hydrous zirconium oxides are amorphous. Upon ageing (e.g., contact with air at room or elevated temperature or treatment with neutral, acidic or alkaline reagents at room or elevated temperature), the primary particles of HZO undergo both intra- and inter-particle condensation reactions, with resultant structural ordering of the precipitates. The severity of the ordering strongly depends on the ageing conditions. However, in general an increased degree of ordering in aged hydrous zirconium oxides accounts for the decrease in sorption capacity, due to loss of free hydroxo groups according to the reactions:Zr(OH)4nH2O→ZrO(OH)2mH2O→ZrO2pH2O (where n,m,p>O).
Therefore, HZO is the most active when freshly precipitated and its ion exchange performance deteriorates with time (especially at elevated temperature). This is a serious drawback of amorphous materials.
Since the hydrous zirconium oxides obtained by the above mentioned methods are usually in the form of a fine powder, difficulties are encountered in handling them in separation processes. To overcome this disadvantage, several approaches have been proposed to make granulated HZO type materials. Among them are the following:
Impregnation of a porous substrate with a zirconium salt, followed by its hydrolysis via treatment with alkali (M. Ozawa et al., J. Mater. Sci. lett., 9, 446 (1990)). The drawback of this approach is the ease in removing HZO from the pores of the carrier;
Granulation of amorphous hydrous zirconium oxides with the use of binders. The drawbacks of the use of binders include: lower ion exchange capacity, deterioration of kinetics of adsorption and possibility of contamination of the product with the binder components;
Sol-gel or gel routes. The sol-gel granulation process, based on the oil-drop principle, includes conversion of ZrO2 sol into spherical granules of hydrous zirconium oxide gel in organic water immiscible media. The gel method, also based on the oil-drop principle, may include neutralization of zirconium salt with hexamethyltetraamine at 70-80° C. (R. Caletka, M. Tympl, J. Radioanal. Chem., 30: 155 (1976)). Spherically granulated amorphous hydrous zirconium oxides prepared via sol-gel and gel routes have high crush strength and good attrition resistance. However, granulated materials have the drawback of a strong dependency between ion exchange performance and moisture content. Amorphous hydrous polyvalent metal oxides regardless of the method of preparation, lose water continuously during storage in air and especially with an increase of heating temperature (See, for example, J. D. Donaldson, M. J. Fuller, J. Inorg. Nucl. Chem., 30, 1083 (1968)).
Highly ordered mesoporous zirconium oxides can be prepared via surfactant-controlled synthesis (U. Ciesla. et al, Chem. Mater., 11, 227 (1999)). The products show high thermal stability up to 400-500° C., which allows the removal of organic surfactants by calcination. Surfactant-controlled synthesis employs the presence of SO4 or PO4 anions as pore and structure-building elements (H. R. Chen et al, Mater. Letters, 51, 187 (2001)).
Different crystalline phases of zirconium oxide can be prepared by thermal treatment of amorphous gels.
Hydrolysis of aqueous zirconium salts under hydrothermal conditions typically results in crystalline modification of HZO. For example, B. Mottet et al. (J. Am. Ceram. Soc., 759, 2515 (1992)) reported the formation of monoclinic HZO by treatment of zirconium oxychloride in the presence of the additives NaOH, Na2CO3, H2SO4, NH4F.
An advantage of crystalline materials is that they are less susceptible to moisture content (water loss) than amorphous sorbents and, as result, are more thermally stable. Ion exchange properties and selectivity of crystalline materials depends on the type of crystal structure. Typically, adsorption capacity of crystalline materials is lower than that of freshly prepared amorphous hydrous zirconium oxide. Another disadvantage of crystalline ion exchangers is poor kinetics of adsorption. Moreover, their powdered form prevents use in column applications.